1. Field of the Invention
The present invention relates to an apparatus and method for transmitting/receiving and relaying signals in a communication system using Double Orthogonal Frequency Division Multiplexing (DOFDM)/Double Orthogonal Frequency Division Multiple Access (DOFDMA) (hereinafter referred to as “DOFDM/DOFDMA communication system”).
2. Description of the Related Art
Next-generation communication systems have developed to provide a variety of high-speed, high-capacity services to Mobile Stations (MSs). Particularly, the next-generation communication systems use Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) to offer the high-speed, high-capacity services. A typical example of the next-generation communication systems using OFDM/OFDMA may include an Institute of Electrical and Electronics Engineers (IEEE) 802.16 communication system, a Mobile Worldwide Interoperability for Microwave Access (WiMAX) communication system, an IEEE 802.11 communication system and the like. The Mobile WiMAX communication system is a communication system based on the IEEE 802.16 communication system, and the IEEE 802.16 communication system represents a communication system using the IEEE 802.16 standard. In addition, the next-generation communication systems actively consider using a relay scheme, especially a cooperative relay scheme, to expand cell coverage, increase data throughput, and increase channel capacity.
Meanwhile, the above-described next-generation communication systems require broad-spectrum resources to offer the high-speed, high-capacity services. However, when the broad-spectrum resources are used, a fading effect becomes significant in a wireless channel due to multipath propagation, and an influence of frequency selective fading occurs even in an actual frequency band.
In this case, therefore, a frequency selective fading channel can be divided into a plurality of frequency flat fading channels using OFDM/OFDMA. Here, a communication system using OFDM/OFDMA will be referred to as an OFDM/OFDMA communication system. In the OFDM/OFDMA communication system, all multi-path signals of a signal transmitted by a signal transmission apparatus should be received within a period of a Cyclic Prefix (CP) at a signal reception apparatus to guarantee orthogonality between sub-carriers. If the signal reception apparatus fails to receive all multi-path signals of the transmission signal within the CP period, Inter-Carrier Interference (ICI) occurs in a signal demodulated by the signal reception apparatus, and the ICI makes the signal reception apparatus difficult to estimate the transmission signal.
Particularly, when the OFDM/OFDMA communication system uses the cooperative relay scheme, a signal received at the signal reception apparatus via a Relay Station (RS) may have a long delay time, compared with a signal received at the signal reception apparatus over a direct path between the signal transmission apparatus and the signal reception apparatus. Here, the time required when the signal transmitted by the signal transmission apparatus is received at the signal reception apparatus via the direct path will be called a ‘direct-delivery delay time’, while the time required when the signal transmitted by the signal transmission apparatus is received at the signal reception apparatus via the RS will be called a ‘relay-delivery delay time’. As described above, a signal receiving delay in the OFDM/OFDMA communication system causes ICI, which deteriorates estimation performance for the transmission signal. Thus, deterioration of signal estimation performance due to the signal receiving delay may become more significant when the OFDM/OFDMA communication system uses the cooperative relay scheme. That is, ICI occurrence probability increases because of the long relay-delivery delay time compared with the direct-delivery delay time, and deterioration of signal estimation performance due to the signal receiving delay may become more significant when the OFDM/OFDMA communication system employs the cooperative relay scheme.
Therefore, there is a need for an apparatus and method for transmitting/receiving and estimating signals such that no ICI may occur in the communication system using a broad spectrum.